The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmits voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
As shown in FIG. 1, which is a 2T1C pixel driving circuit employed for AMOLED, comprising a first thin film transistor T10, a second thin film transistor T20 and a capacitor C. The first thin film transistor T10 is a switch thin film transistor, and the second thin film transistor T20 is a drive thin film transistor, and the capacitor C is a storage capacitor. Specifically, a gate of the first thin film transistor T10 is electrically coupled to a scan signal Scan, and a source is electrically coupled to a data signal Data, and a drain is electrically coupled to a gate of the second thin film transistor T20 and one end of the capacitor C; a source of the second thin film transistor T20 is electrically coupled to a power source positive voltage VDD, and a drain is electrically coupled to an anode of an organic light emitting diode D; a cathode of the organic light emitting diode D is grounded; the one end of the capacitor C is electrically coupled to the drain of the first thin film transistor T10, and the other end is electrically coupled to the source of the second thin film transistor T20. As the AMOLED displays, the scan signal Scan controls the first thin film transistor T10 to be activated, and the data signal Data enters the gate of the second thin film transistor T20 and the capacitor C via the first thin film transistor T10. Then, the first thin film transistor T10 is deactivated. With the storage function of the capacitor C, the gate voltage of the second thin film transistor T20 can remain to hold the data signal voltage to make the second thin film transistor T20 to be in the conducted state to drive the current to enter the organic light emitting diode D via the second thin film transistor T20 and to drive the organic light emitting diode D to emit light.
The aforesaid 2T1C pixel driving circuit employed for AMOLED according to prior art is highly sensitive to the threshold voltage drift of the drive thin film transistor. Along with the threshold voltage drift of the drive thin film transistor, the change of the current flowing through the organic light emitting diode is very large. As shown in FIG. 2, the 2T1C pixel driving circuit employed for AMOLED according to prior art is tested. As the threshold voltage of the drive thin film transistor respectively drifts ±0.5V relative to −0.2V, the change ratios of the currents flowing through the organic light emitting diode OLED exceed 40.66%, and even up to 79.39% under several different data signal voltage conditions. Thus, the current flowing through the organic light emitting diode is unstable and the brightness of the organic light emitting diode is very nonuniform, which extremely affect the display effect of the pictures. For solving the aforesaid issue, it is necessary to add a compensation circuit to each of the pixels. The compensation means that the compensation has to be implemented to the threshold voltage of the drive thin film transistor in each pixel to make the current flowing through the organic light emitting diode irrelevant with the threshold voltage.